Study on temporal variations of shear—wave splitting in the Chiayi area, aftershock zone of 1999 Chichi earthquake, Taiwan
1999年台湾集集地震余震区——嘉义地区地震的剪切波分裂参数随时间变化的研究

Based on the data from three seismic stations CHY, CHN2 and CHN8 of Taiwan Central Weather Bureau Seismic Network (CWBSN), the systematic analysis eth od (SAM) of shear-wave splitting has been used to do the near-source stress-f orecast research of temporal preseismic variations of polarization directions an d time delays in nearly 22 months before 1999 ML 6.4 and ML 6.0 earthquakes in the Chiayi area, the aftershock zone of MW7.6 Chichi, Taiwan earthquake of September 20, 1999.The result shows that the normal polarization directions of station CHY, CHN2 and CHN8 are approximately EW, in good agreement with the direction of maximum compressional stress, suggesting that the anisotropy in this area is controlled by local tectonic stress field. The results strongly indicate that in the stress critical stage, the polarization 90°-flips occurred frequently, and that there were abrupt decreases in time delays shortly before the Chichi mainshock and two Chiayi earthquakes. These can be considered as two precursors of stress-forecast. We think that the temporal variations of shear-wave splitting are of great importance, since they provide information about source stress processes and critical warning information before large earthquakes.